This invention relates to a method of controlling an air-fuel ratio, and more particularly to a method of controlling an air-fuel ratio, suitable for use in an internal combustion engine for a vehicle, having an electronically controlled fuel injection device.
In an electronically controlled fuel injection device, a basic fuel injection time duration TP is computed on the basis of an engine speed NE detected by a rotational speed sensor and an intake air flowrate Q detected by an intake air flow sensor, and various correction are applied to the basic fuel injection time duration TP in accordance with the engine operating conditions so as to compute a final fuel injection time duration .tau.. A fuel injection valve is opened to inject the fuel for the final fuel injection time duration .tau..
On the other hand, in the fuel injection control device of the type described, in which CO, HC and NO.sub.x are to be simultaneously removed for the exhaust gas emission control measure, it is desired to control the air-fuel ratio in the vicinity of the stoichiometric air-fuel ratio from the viewpoint of the effective removal of the above-mentioned three contents. Therefore, an oxygen sensor is provided in the exhaust gas path, and, under predetermined condition, the feedback correction coefficient FAF is computed so that the air-fuel ratio can approach the vicinity of the stoichiometric air-fuel ratio in accordance with an air-fuel ratio signal from the oxygen sensor, whereby the air-fuel ratio is feedback-controlled.
In the electronically controlled fuel injection device wherein the above-described feedback control of the air-fuel ratio, the air-fuel ratios under the predetermined conditions during the above-described feedback control are learned to compute learning correction coefficient FG in order to compensate a difference in the air-fuel ratio due to the variability of parts, compensate the air-fuel ratio for the running of the vehicle in the highlands (for the high altitude) and compensate a variation in the air-fuel ratio due to change of the intake air flow sensor with time.
For example, the final fuel injection time duration .tau. is obtainable through the following equation. EQU .tau.=TP.times.FAF.times.FG.times.K
where K is a correction coefficient determined by water temperature, intake air temperature and the like.
In learning the aforesaid air-fuel ratio, it must be taken in consideration that the fuel, which has evaporated from a fuel tank and has been accumulated in a canister (hereinafter referred to as the "evaporated fuel"), is fed to a combustion chamber under predetermined condition including that at least the throttle valve is not fully closed, and thus the air-fuel ratio becomes rich temporarily. The influence by the evaporated fuel upon the air-fuel ratio is as shown in FIG. 1. In an extreme case, the intake air flowrate Q becomes about 10% rich even in a region of a high air flowrate as high as 100 m.sup.3 /h. In consequence, if the operation of the vehicle is stopped immediately after the change in the air-fuel ratio due to the evaporated fuel as described above is learned, then the air-fuel ratio would become excessively lean when the vehicle is started again, thus presenting the disadvantage of lowered startability. For this reason, there is no need to learn the air-fuel ratio, which has become rich due to the evaporated fuel.
The compensation of the air-fuel ratio for the aforesaid high altitude prevents the air-fuel ratio from becoming richer. More specifically, since the higher the altitude is, the lower the air density becomes, the air-fuel ratio becomes richer when the vehicle runs at the highlands. Therefore, in the compensation for the high altitude, the fuel injection rate is adapted to get less as the altitude becomes higher. The influence by the altitude of the highland upon the air-fuel ratio is substantially constant irrespective of the intake air flowrate as shown in FIG. 2. Because of this, in a region other than the region where the throttle valve is fully closed, it is difficult to attribute the air-fuel ratio being rich to whether the evaporated fuel or the altitude of the highland.
On the other hand, when the intake air flow sensor is obstructed due to a change with time, as indicated by a curve B in FIG. 3, the less the intake air flowrate in any region is, the more influence to the air-flow rate in such a region is given.
According to the air-fuel ratio learning control method proposed by the inventors of the present invention, an intake air flowrate is divided into 16 flowrate regions Q.sub.1 -Q.sub.16 for example. When the air-fuel ratio is on the lean side of the stoichiometric air fuel ratio, a predetermined number is added to obstruction compensating learning correction coefficients FGQ.sub.c for the latest flowrate region Q.sub.c, FGQ.sub.c-1 for a flowrate region before Q.sub.c and FGQ.sub.c-1 for a flowrate region after Q.sub.c, and, when the air-fuel ratio is on the rich side, the predetermined number is subtracted therefrom. In addition to this calculation, a value obtained by dividing the total sum of the obstruction learning correction coefficients FGQ.sub.1 -FGQ.sub.16 for all of the flowrate regions Q.sub.1 -Q.sub.16 is made to be an altitude compensating learning correction coefficient FHAC. Then, in consideration of the influence by the evaporated fuel, the obstruction compensating learning correction coefficient FGQ is guarded within a predetermined range centered about a step-shaped guard line G as shown in FIG. 3.
In the above-described air-fuel ratio learning control thus proposed, if the operation is performed only in the specific flowrate region, such a disadvantage is presented that the obstruction compensating learning correction coefficient FGQ and the altitude compensating learning correction coefficient FHAC are learned only in the specific flowrate region. In consequence, there is such a possibility that, when a motor vehicle provided with such a air-fuel ratio learning control goes up to highlands only in the large flowrate region for example, the learning cannot be performed in the small flowrate region. Accordingly, the air-fuel ratio becomes over-rich due to the high altitude, so that the engine may not start.
On the other hand, in such a learning control, in order to obviate the influence by the evaporated fuel, the obstruction compensating learning correction coefficient FGQ is limited as indicated by a regulated value G as shown in the aforesaid FIG. 3. However, the air-fuel ratio is influenced by the evaporated fuel within the range defined the curve B and the line G. Further, since the above-described regulated value is set as shown in FIG. 3, the obstruction compensating learning correction coefficient FGQ cannot be regulated in accordance with the characteristics of obstruction of the air flow meter as indicated by a curve B in FIG. 3. Furthermore, after the obstruction compensating learning correction coefficient FGQ is regulated by the regulated value G in all of the flowrate regions, the altitude compensating cannot be satisfactorily effected.